I'm a big Bottom Line fan
It's a 60wt poly bobbin thread. My old Bernina will take it top and bottom. Sews like a dream, lies flatter than any other thread.
My Jems won't take it as a top thread. With them I use Aurifil 50wt cotton on top, BL in bobbin. Aurifil seams lie flatter than any other cotton 50wt.
http://www.superiorthreads.com